How to update my iPhone 6s iOS?

To update to the most recent, compatible version of iOS 15.7.5, please see: Update your iPhone, iPad, or iPod touch - Apple Support

And: If your iPhone, iPad, or iPod touch won't update - Apple Support

If the above Apple articles do not help, please post back with relevant information. 

The iPhone 6s cannot update to iOS 16, if that is what you are asking about.

To see if another update might be available for your phone, make sure you have a good Wi-Fi Internet connection and the battery is fully charged or connected to the power adapter.

On the Home Screen, tap Settings > General > Software Update